Observed regional variations in breast cancer incidence have been a source of public health concern as well as traditionally a source of hypothesis generation for factors which might be implicated in disease etiology. Rate differences within California are similar to differences noted on the national scale. This proposal is designed to explore factors that may explain the observed regional variations within California. The specific aims of the study are to: use a geographic information system (GIS) and grouped data sources to evaluate the potential influence of sociodemographic factors (e.g., income, education), environmental toxicants(e.g., pesticides, automobile exhaust), and established risk factors (e.g., family history, reproductive risk factors) on regional differences in breast cancer incidence rates in California; further evaluate regional associations identified in Aim 1 by incorporating individual risk factor information from a large, prospective cohort study of California teachers specifically designed to study risk factors for breast cancer; and conduct a biomarker substudy targeted at exploring urban/rural variations in biological markers of two specific exposures of emerging interest in breast cancer research (i.e, phytoestrogens and polycyclic aromatic hydrocarbons). Statewide grouped analyses will rely on multiple sources of data including the largest cancer registry in the world, two statewide risk factor surveys, multiple statewide environmental datasets, and census data. Individual-level risk factor information will be derived from the 133,400 female teachers enrolled in the California Teacher Studies (CTS) cohort who are geographically dispersed throughout the state. The biomarker substudy will be nested within this cohort and will be conducted on 130 urban and 130 rural CTS members to evaluate both dietary and environmental markers for exposures potentially related to regional variations in breast cancer. These multiple sources of data, coupled with the use of a GIS system, offer a unique opportunity to integrate individual risk factor information with potentially important community-level factors in a way that no other study has done to date. Furthermore, the biomarker substudy will allow us to evaluate two emerging and intriguing hypotheses regarding breast cancer etiology in the context of urban/rural variations in breast cancer rates.